1. Field of the Invention
The present invention relates to a micro-Raman spectroscopy system used for non-destructive analysis of foreign materials adhering to surfaces of semiconductor wafers.
2. Description of the Related Art
Semiconductor devices, such as VLSI and ULSI devices, have been fabricated with decreasing dimensions and higher scales of device integration, and a new generation of technology is introduced every three or four years. With this trend, the fabrication yield tends to deteriorate. Accordingly, there is a demand for improvement of the fabrication yield to reduce the fabrication cost.
It is said that low yields in fabrication of VLSI and ULSI devices are caused by microscopic foreign materials which adhere to surfaces of semiconductor wafers during fabrication and hinder correct printing of patterns of integrated circuits. Therefore, in order to improve the yield, it is important to identify the foreign materials and to prevent their adhesion.
A micro-Raman spectroscopy system that is a combination of an optical microscope and a laser Raman spectrometer has enjoyed wide acceptance as a means for identifying the components of microscopic samples. In particular, laser light is introduced to the microscope. The light is focused on a microscopic sample placed under the microscope. Scattering light from the microscopic sample is gathered by the microscope and introduced to the Raman spectrometer. In this way, Raman spectra are obtained.
Scattering light from the sample includes elastic scattering light and inelastic scattering light. Generally, elastic scattering light is known as Rayleigh scattering light, while inelastic scattering light is known as Raman scattering light. Since Raman scattering light is about six orders of magnitude weaker (i.e., 1/1,000,000) than Rayleigh scattering light, the Rayleigh scattering light must be removed by an optical filter and spectrally resolved to obtain a Raman spectrum. The chemical components of the sample can be identified by analyzing the Raman spectrum.
An available means for observing the morphologies of foreign materials on a semiconductor wafer is to use an optical review microscope system having so-called review capability. Specifically, the coordinate values of a foreign material have been already measured. The coordinates are reproduced under a microscope, and an observation of the foreign material is made. A sample stage on which a sample is placed is controlled by a computer. The coordinate values of the foreign material are entered into the computer. The sample stage is moved into a desired position to bring the foreign material immediately under the optical microscope. In this way, the morphologies of the foreign material can be observed.
Also, a method using a review SEM (scanning electron microscope) instead of an optical microscope has been invented. This enables observation of finer morphologies of foreign materials. This system is generally known as a review SEM system.
The prior art micro-Raman spectroscopy system of the construction described above has the problem that it has no review capability. Therefore, it is quite difficult to bring a foreign material on a wafer into the measuring position under a microscope. Silicon wafers are now taken as an example. Wafers generally have diameters of as large as 8 inches (200 mm). It is very difficult to search for micrometer-to-submicrometer foreign materials on such wafers only with a microscope. In the field of semiconductor fabrication, therefore, there are dedicated foreign material inspection systems for searching for foreign materials and displaying their coordinate positions and sizes. However, a conventional micro-Raman spectroscopy system having no review capability cannot make use of results of inspections of foreign materials (e.g., positional information) obtained by a dedicated foreign material inspection system. As a result, the features of Raman spectroscopy having excellent component-analyzing capabilities cannot be utilized for inspections of foreign materials on semiconductor wafers.
On the other hand, where the coordinates of the positions of foreign materials obtained by a dedicated foreign material inspection system and information about their sizes should be utilized, the user must rely on an optical review microscope system or on a review SEM system. In particular, information about foreign materials on a wafer is recorded by a foreign material inspection system. The wafer is placed on a sample stage according to the information about the foreign materials. Each foreign object is moved into the field of view of the optical review microscope or review SEM and observed as a microscope image. The name of the substance of the foreign material is estimated from the results of observation of the morphology. However, limitations are imposed on this method of estimating the substance name only with observation of the morphology. Therefore, in review SEM, for example, an energy dispersive X-ray spectrometer (EDS) is used in combination. Atomic names are judged from the characteristic X-rays arising from the foreign material. Relying on EDS is more effective than relying on only observation of morphologies. However, it is still impossible to judge organic materials and light elements.
In view of the foregoing, it is an object of the present invention to provide a micro-Raman spectroscopy system which is capable of making effective use of the unique analyzing capabilities of Raman spectroscopy and still capable of employing information about foreign materials obtained by a separate foreign material inspection system.
This object is achieved in accordance with the teachings of the present invention by a micro-Raman spectroscopy system for observing morphologies of a foreign material on a wafer, the micro-Raman spectroscopy system having an optical microscope for observing the morphologies of the foreign material on the wafer, a sample stage having a function of moving the foreign material into the field of view of the optical microscope, a wafer transport mechanism for taking the wafer from a wafer cassette and setting the wafer on the sample stage, at least one Raman-exciting laser for exciting chemical bonds in the foreign material on the wafer, at least one Raman analysis optical system, a Raman spectrometer, and a control means for controlling the sample stage, the wafer transport mechanism, and the Raman spectrometer. The Raman analysis optical system brings the optical axis of the Raman-exciting laser into coincidence with the optical axis of the optical microscope to direct laser light to the foreign material on the wafer. The Raman analysis optical system also acts to take Raman scattering light out of the optical axis of the optical microscope, the Raman scattering light being emitted from the foreign material excited with the laser light. The Raman spectrometer spectroscopically analyzes the Raman scattering light taken from the foreign material on the wafer by the Raman analysis optical system. This micro-Raman spectroscopy system is characterized in that the control means drives the sample stage according to positional information about the foreign material previously obtained by a separate foreign material inspection system, the sample stage having the wafer set thereon. Thus, an image of the foreign material is reproduced within the field of view of the optical microscope.
In one feature of this embodiment of the present invention, the optical microscope described above is equipped with a CCD camera.
In another feature of this embodiment, the optical path between the Raman-exciting laser and the foreign material on the wafer includes at least one of a reflecting mirror and optical lenses.
In a further feature of this embodiment, the optical path between the Raman analysis optical system and the Raman spectrometer is composed of at least optical fiber.
In a still other feature of this embodiment, the control means described above is a computer.
In a yet other feature of this embodiment, the micro-Raman spectroscopy system has a function of searching a built-in database for at least the name of a substance forming the foreign material, using a Raman spectrum presently obtained from the foreign material.
The present invention also provides a micro-Raman spectroscopy system having an optical microscope for observing morphologies of a foreign material on a wafer, a sample stage having a function of moving the foreign material into the field of view of the optical microscope, a wafer transport mechanism for taking the wafer from a wafer cassette and setting the wafer on the sample stage, at least one Raman-exciting laser for exciting chemical bonds in the foreign material on the wafer, at least one second (non-Raman) exciting laser for exciting the foreign material on the wafer, a selector means for passing only one of laser light from the Raman-exciting laser and laser light from the second exciting laser, at least two spectroscopic analysis optical systems, a Raman spectrometer, a non-Raman spectrometer, and a control means for controlling the sample stage, the wafer transport mechanism, the Raman spectrometer, and the non-Raman spectrometer. The spectroscopic analysis optical systems bring the optical axes of the exciting lasers including the Raman-exiting laser into coincidence with the optical axis of the optical microscope and directs laser light to the foreign material on the wafer. The spectroscopic optical systems also act to take excited light out of the optical axis of the optical microscope, the excited light arising from the foreign material excited with the laser light. The Raman spectrometer spectroscopically analyzes the excited light taken from the foreign material on the wafer by the Raman analysis optical system. The non-Raman spectrometer spectroscopically analyzes excited light taken from the foreign material on the wafer by the spectroscopic optical system other than the Raman analysis optical system. This micro-Raman spectroscopy system is characterized in that the control means drives the sample stage according to positional information on the foreign material previously obtained by a separate foreign material inspection system, the sample stage having the wafer set thereon. Thus, an image of the foreign material is reproduced within the field of view of the optical microscope.
In one feature of this embodiment, the optical microscope described above is equipped with a CCD camera.
In another feature of this embodiment, the optical path between the Raman-exciting laser and the foreign material on the wafer includes at least one of a reflecting mirror and optical lenses.
In a further feature of this embodiment, the second (non-Raman) exciting laser includes at least one of a photoluminescence-exciting laser and a fluorescence-exciting laser.
In yet another feature of this embodiment, the selector means for switching the laser light is a movable mirror.
In still another feature of this embodiment, the optical path between the Raman analysis optical system and the Raman spectrometer is composed of at least optical fiber.
In an additional feature of this embodiment, the optical path between the spectroscopic optical system other than the Raman analysis optical system and the non-Raman spectrometer is composed of at least optical fiber.
In a yet additional feature of this embodiment, the spectroscopic analysis optical system other than the Raman analysis optical system includes at least one of a photoluminescence-exciting laser and a fluorescence-exciting laser.
In a yet further feature of this embodiment, the second, non-Raman spectrometer includes at least one of a photoluminescence spectrometer and a fluorescence spectrometer.
In another feature of this embodiment, the control means is a computer.
In another feature of this embodiment, the micro-Raman spectroscopy system has a function of searching a built-in database for at least the names of substances forming the foreign material using various spectra of the foreign material.
Other objects and features of the invention will appear in the course of the description thereof, which follows.